Modeling magnetotelluric tippers on a sphere with particular emphasis on ocean induction effect and effect from oceanic sediments

Abstract

In the past three decades, an immense amount of long-period magnetic field data (with periods of measurements from months to years) has been collected worldwide, either inland or at sea bottom. This makes it tempting to estimate from the data magnetotelluric (MT) vertical transfer functions -tippers - and further probe with them the three-dimensional distribution of electrical conductivity on a global or semi-global/continental scale. Such problem setup requires modeling MT responses in spherical geometry. It is known that MT impedances in spherical coordinates can be modeled using different polarizations of a uniform external magnetic field. As for tippers, one needs another type of excitation because the uniform external magnetic field of any polarization contains a non-zero radial component. In the paper, we elaborate a model of the source, which leads to valid tippers on a whole sphere. We also present an accurate and computationally efficient solver to calculate the electromagnetic field and responses in a spherical shell or a part thereof. The solver, based on nested integral equations, was used to calculate high-resolution tippers regionally and globally, taking into account realistic oceans and Earth’s conductivity. In particular, we investigate an effect in tippers from bathymetry and oceanic sediments, both at the Earth's surface and sea bottom sediments.